The cancer treatment technique of therapeutic hyperthermia currently suffers from too little knowledge of the specific effects on the treated tissues for accurate prognosis. Currently, no simple technique exists to obtain the needed data. SPECIFIC AIMS; A study will be conducted to establish the feasibility of using in vivo measurements of tissue electrical impedance for determining and monitoring the effect of clinical hyperthermia. Hypothesis: When a tissue is subjected to therapeutic hyperthermia, the changes representing actual cell damage will be reflected in the extent of membrane damage to those cells. Since the low frequency electrical impedance of tissues is predominently due to the characteristics of the cell membranes, the in vivo measured impedance will provide data from which the clinical effect of hyperthermia can be determined. To test this hypothesis, the project will determine whether the established general characteristics of tissue response to hyperthermia can be ascertained from in vivo measurements of the electrical impedance during and after hyperthermia. EXPERIMENTAL DESIGN AND METHODS: The tissue's complex electrical impedance will be measured in the frequency range from 10Hz to 100MHz, during and following hyperthermia application, using an impedance analyzer, small implantable electrodes in vivo, and electrodes or parallel plate capacitor geometry for excised tissue. The experimental outline follows: 1. determination of electrode variables; 2. mapping of temperature distributions; and 3. hyperthermia experiments in excised tissues and in vivo. For in vivo measurements the thigh muscles of adult rats and tumors grown in nude mice will be used. Uniform therapeutic temperatures will be quickly reached and the electrical impedance periodically measured during and after the hyperthermia, in regions of known temperature. Temperature will be held at +/-.5deg C.